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$parent = 'parent_field';
$field = 'sub_field';
$post_type = 'post';
if( function_exists( 'have_rows' ) ) : 
  if( have_rows( $parent, 'option' ) ): 
    while ( have_rows( $parent, 'option' ) ) : 
      the_row();
      // Stuff
    endwhile;
  endif;
else :
  $rows = $wpdb->get_results( $wpdb->prepare( " SELECT * 
                                                FROM {$wpdb->prefix}posts 
                                                INNER JOIN {$wpdb->prefix}postmeta 
                                                ON ({$wpdb->prefix}posts.ID = {$wpdb->prefix}postmeta.post_id) 
                                                WHERE {$wpdb->prefix}posts.ID = $post->ID
                                                AND {$wpdb->prefix}posts.post_type = $post_type
                                                AND {$wpdb->prefix}postmeta.meta_key 
                                                LIKE %s", $parent . '_%_' . $field ) );
  if( $rows ) :
    foreach( $rows as $row ) :
      preg_match( '_([0-9]+)_', $row->meta_key, $matches );
      $meta_key = $parent . '_' . $matches[0] . '_' . $field;
      $post_meta = get_post_meta( $row->post_id, $meta_key, true );
      // Stuff
      echo $post_meta; // The Field
    endforeach;
  endif;
endif;